1. Field of the Invention
The present invention relates to a laminated type dielectric filter comprising input electrode, output electrode, earth electrode, coupling electrode and resonance electrodes composing a resonator, said electrodes being laminated with each other with interposing dielectric layers therebetween, and more particularly to a laminated type dielectric filter including a strip line resonator.
2. Related Art Statement
Heretofore, there have been proposed such a laminated type dielectric filter including a strip line resonator in, for instance Japanese Patent Application Publication Kokai Hei 6-120703. FIGS. 1 and 2 illustrate this known laminated type dielectric filter. Upon manufacturing such a laminated type dielectric filter, a plurality of dielectric substrates having various kinds of electrodes formed by conductive patterns applied thereon are stacked one another, and after covering upper and lower surfaces and side walls of a stacked assembly except for portions corresponding to input and output electrodes with conductive layers, the assembly is heated to sinter the dielectric substrates. Therefore, in the finally manufactured filter, interfaces between successive dielectric layers are not existent and the various kinds of electrodes are embedded within a dielectric block at given relative positions. However, for the sake of explanation, in the present specification, the dielectric block is shown to be divided into a plurality of dielectric layers each having various kinds of electrodes formed on surfaces thereof as shown in FIG. 1. FIG. 2 depicts an outer appearance of the sintered body of the laminated type dielectric filter.
The known laminated type dielectric filter shown in FIG. 1 comprises first to fourth dielectric layers 1-4. On outer surfaces of the first and fourth dielectric layers 1 and 5 are formed first and second earth electrodes 5 and 6, and on an inner surface of the first dielectric layer 1 are formed an input electrode 7 and an output electrode 8 such that one ends of these electrodes are exposed at side walls of the laminated type dielectric filter. As shown in FIG. 2, these input and output electrodes 7 and 8 are connected to input and output terminals 7a and 8a, respectively provided on the side wall of the laminated type dielectric filter.
On a surface of the second dielectric layer 2, are formed resonating elements 9 and 10 constructed by strip line electrodes of strip line resonators such that one ends of these resonating elements are connected to an earth electrode 11 (refer to FIG. 2) including the above mentioned earth electrodes 5 and 6 at the side wall of the laminated type dielectric filter, and the other ends of the resonating elements are opened.
Furthermore, on a surface of the third dielectric layer 3 is formed a coupling electrode 12 such that the coupling electrode is overlapped with the resonating elements 9 and 10 via the dielectric layer 3, said coupling electrode coupling the resonating elements inductively. On the first and third dielectric layers 1 and 3 are formed interlayer earth electrodes 13-16 such that these electrodes are overlapped with the open ends of the resonating elements 9 and 10. Ends of these interlayer earth electrodes 13-16 are connected to the earth electrode 11 at the side wall of the laminated type dielectric filter.
In such a laminated type dielectric filter, there are provided four electrostatic capacitances between the resonating element 9 and the interlayer earth electrodes 13 and 15 as well as between the resonating element 10 and the interlayer earth electrodes 14 and 16, and these electrostatic capacitances are connected in parallel with the resonating elements 9 and 10. Therefore, a resonance frequency can be lowered without decreasing a length of the resonating elements.
In the known laminated type dielectric filter, electrostatic capacitance obtained between the coupling electrode 12 and the resonating elements 9 and 10 is connected in parallel with the inductance due to the electromagnetic coupling between the adjacent resonators, and thus a parallel resonance circuit of the electrostatic capacitance and inductance is connected between the adjacent resonators. Since an impedance of this parallel resonance circuit changes from inductive to capacitive at a parallel resonant frequency, the coupling between the resonators may be inductive or capacitive by adjusting the electrostatic capacitances formed between the coupling electrode and the adjacent resonators, respectively. In case of the inductive coupling, it is possible to obtain a filter having an attenuation peak on a higher frequency side to the pass band, and when the electrostatic capacitance is increased, the pass band is narrowed. In the capacitive coupling, it is possible to obtain a filter having an attenuation peak on a lower frequency side to the pass band, and when the electrostatic capacitance is increased, the pass band is widened. When a center frequency of the filter is lowered, the inductive coupling between the adjacent resonators becomes stronger, and therefore in the filter having the attenuation peak on the lower frequency side, the pass band is narrowed, while in the filter having the attenuation peak on the higher frequency side, the pass band is broadened. However, in the latter case, the pass band can be adjusted by increasing the electrostatic capacitance.
In the laminated type dielectric filter having the strip line resonators, there are provided the input electrode 7 for connecting the input terminal 7a to the first stage resonator 9 by means of the capacitance and the output electrode 8 for connecting the final stage resonator 10 to the output terminal 8a via the capacitance. In such an input and output system by means of the capacitance, the input impedance and output impedance are determined by the capacitances, and therefore by increasing the capacitances, the input impedance and output impedance can be adjusted over a wider range and it is possible to realize a required filter easily.
In accordance with the miniaturization and variety of wireless communication systems, the known laminated type dielectric filter illustrated in FIG. 1 is also required to be small in size and light in weight, and further it is required to manufacture filters having various frequency band properties. For instance, the laminated type dielectric filter is installed within a portable type telephone set. Since the portable type telephone set has been earnestly required to be small in size and light in weight, a further small laminated type dielectric filter is required for the portable type telephone set. In order to make the laminated type dielectric filter be small in size and light in weight, those electrodes are required to be small in size, and thereby the overlapping parts between the electrodes have small areas inevitably, which results in the capacitances between the resonating elements 9, 10 and the interlayer earth electrodes 13 to 16 being small, the capacitances between the resonating elements 9, 10 and the coupling electrode 12 being small, and the capacitances between the resonating elements 9, 10 and the input and the output electrodes 7, 8 being small. Thus, a desired frequency characteristics can not be obtained. If the filter is made be small in size, the resonating elements 9, 10 are small in size, and thereby the capacitances between the resonating elements and the interlayer earth electrode is decreased and the resonating frequency is increased.
Moreover, if the coupling electrode 12 is small in size, the capacitances between the resonating elements 9, 10 and the coupling electrode 12 is decreased, so that when the coupling between the resonators is capacitive, the pass band of the filter is too narrowed, and when the coupling is inductive, the pass band of the filter is too broadened. Thus, an adjustable range of the pass band becomes very narrow and the laminated type dielectric filter having a required pass band width could not be obtained.
Furthermore, if the capacitances between the resonating elements 9, 10 and the input and the output electrodes 7, 8 are small, a return loss could not be large, so that it is difficult to obtain the filter having a low insertion loss.
As above mentioned, in accordance with the miniaturization and variety of the wireless communication systems, the conventional laminated type dielectric filter shown in FIG. 1 has been required to be small in size and have various pass bands. In order to satisfy a requirement of decreasing the pass band frequency without increasing a size and a configuration of the filter, the capacitances between the resonating elements 9, 10 and the interlayer earth electrodes 13 to 16 have to be further increased. If the capacitances between the resonating elements 9, 10 and the interlayer earth electrodes 13 to 16 can be increased, the laminated type dielectric filter having a higher frequency can be easily obtained.
In order to increase the capacitances between the resonating elements 9, 10 and the interlayer earth electrodes 13 to 16, it has been proposed to increase surface areas of overlapping parts of these electrodes. In order to realize it, a width of the open ends of the resonating elements 9, 10 overlapping the interlayer earth electrodes 13 to 16 has to be increased as shown in FIG. 3. However, since this method of increasing the overlapping surface area between the resonating elements 9, 10 and the interlayer earth electrodes 13 to 16 has a limitation, the frequency band can be adjusted only within a narrow range.
Moreover, in another method of increasing the capacitances between the resonating elements 9, 10 and the interlayer earth electrodes 13 to 16, a thickness of the dielectric layers 2, 3 provided therebetween is decreased. However, if the dielectric layers have a small thickness, manufacturing cost of the laminated type dielectric filter might be increased due to dielectric breakdown, decrease in physical strength, and difficulty in handling during manufacturing processes.
Furthermore, JP A 7-226602 proposes another method of increasing the capacitances between the resonating elements 9, 10 and the input and the output electrodes 7, 8, in which a resonating element of an input stage strip line resonator is sandwiched with two input electrodes and a resonating element of an output stage strip line resonator is sandwiched with two output electrodes. However, when interlayer earth electrodes are provided for reducing the filter size and a coupling electrode is provided for improving attenuation characteristics, there is no sufficient space for arranging the two input electrodes and the two output electrodes on respective sides of the resonating elements.
Moreover, JP A 4-43703 discloses a symmetric type strip line resonator. In such a symmetric type strip line resonator, capacitances between resonating elements and interlayer earth electrodes can be increased by stacking the resonating elements in a laminated fashion. In this known filter, however, in order to resonate a pair of resonating elements composing the symmetric type strip line resonator at the same frequency with the same phase, these resonating elements are connected each other via a conductor passing through a through hole formed therebetween. It is, however, difficult to manufacture a symmetric type strip line resonator having such a structure, which results in a high cost.
The above mentioned JP A 4-43703 suggests to constitute a laminated type dielectric filter by aligning plural symmetric type strip line resonators in a direction perpendicular to the stacking direction. However, no layer except for a dielectric layer is arranged between a pair of resonating elements, and there are no teaching how to constitute a practical laminated type dielectric filter.